Multimode fiber optical fiber transmission system with offset launch single mode long wavelength vertical cavity surface emitting laser transmitter

ABSTRACT

A multimode fiber optical fiber transmission system includes an improved configuration for launching a single mode long wavelength transmission signal into existing multimode optical fiber networks. More specifically, the invention utilizes new single mode long wavelength VCSEL devices to realize a novel transmitter/transceiver for multimode fiber links where offset launch with controlled mode conditioning is achieved without the use of a mode conditioning patchcord, and in some embodiments, without the use of any collecting or focusing elements.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

This application claims the priority benefit of U.S. Provisional PatentApplication No. 60/472,278, filed May 21, 2003, entitled “MULTIMODEFIBER OPTICAL FIBER TRANSMISSION SYSTEM WITH OFFSET LAUNCH SINGLE MODELONG WAVELENGTH VERTICAL CAVITY SURFACE EMITTING LASER TRANSMITTER”.

BACKGROUND OF THE INVENTION

The present invention relates to optical fiber transmission systems andmore particularly to an improved configuration for launching a singlemode long wavelength transmission signal into existing multimode opticalfiber networks.

Much of the installed based of optical fiber consists of multimodefiber. The problem addressed by this invention is that a significantamount of the installed multimode fiber base consists of inferiorquality fiber where the bandwidth associated with the Over-Fill Launch(OFL) condition is not realized. In fact, approximately 10% of installedfiber can suffer bandwidth collapse with long wavelength lasertransmitters when launched on axis or with a typical offset of 3 to 7 umfor a multimode fiber connector. This has been a serious problem in thepast, and has been the focus of many experts in this area for the past15 years. As early as 1991, the U.S. Pat. No. 5,077,815 to Yoshizawadisclosed the general concept of launching the transmission signal downthe multimode fiber offset from the optical axis of the core. Yoshizawautilized a single mode optical fiber coupled to a laser diode, or LED,for launching the transmission signal into the core of the multimodefiber. The single mode fiber was butt-coupled to the end face of themultimode fiber with the core of the singe mode fiber offset from theoptical axis of the multimode fiber. Alternate configurations usinglenses to focus the optical radiation from the single mode fiber to themultimode fiber were also disclosed. The theoretical basis for therealized improvement in bandwidth was believed to be a preferentialexcitation of higher order mode groups as opposed to lower order modegroups.

In 1995 the U.S. Pat. No. 5,416,862 to Haas identified that adding anangle to either a center launch or an offset launch would furtherpreferentially excite higher order mode groups traveling in themultimode fiber and would also increase bandwidth.

The most recent work in this area can be found in the U.S. Pat. No.6,064,786 to Cunningham et al which describes a theoretical model thatdistinguishes preferential excitation of only mid-order modes. Theprimary configurations claimed in Cunningham use a long wavelength (1300nm) Fabry Perot edge emitter diode, and require the use of a multimodecollecting fiber for collecting optical radiation from the diode. Loworder mode groups are excited in the collecting multimode fiber and arelaunched into a conducting multimode fiber offset from the optical axisof the conducting multimode fiber where they preferentially excitemid-order mode groups. One alternate configuration disclosed in theCunningham '768 patent briefly describes the experimental testing of ashort wavelength (850 nm) VCSEL operating in a single transverse modeand launching the VCSEL radiation into the multimode fiber offset fromthe optical axis of the core. However, practical use of the system wasdiscounted due to power limitations of the VCSEL. The short wavelengthVCSEL described in Cunningham would had to have been operated at a lowpower in order for it to lase in only a single transverse mode. For themost part, the '768 Cunningham patent seems to set forth an explanationof the theoretical reasons “why” offset launch provides betterbandwidth.

Another practical solution for providing an offset launch into multimodefiber communication systems has been addressed by the GbE standardsbodies by specifying a mode conditioning patchcord that is used toguarantee a known offset in the range 17 to 23 um for 62.5 um multimodefiber and 10 to 16 um for 50 um multimode fiber where the reduced numberof excited modes guarantees satisfactory performance. The modeconditioning patchcord is an extra component used between a conventionalsingle mode fiber transmitter (transceiver) and the multimode fibersystem that guarantees the optimum launch condition. Further details ofthe multimode fiber patch cord configurations can be found in the U.S.Pat. No. 6,304,352 to Cunningham.

A further improved solution to this problem has been found and is thebasis for the present invention. The fiber bandwidth demonstrated forthe present invention is far better than predicted, and this has openedup a new commercial opportunity for data links on multimode fiber byextending the distances and data transfer speeds that can beaccommodated.

SUMMARY OF THE INVENTION

In recent years, significant effort has gone into developing LongWavelength Vertical Cavity Surface Emitting Lasers (VCSEL's). One longwavelength VCSEL material system specifically in current development isbased on InGaAsN technology, but other material systems are alsopossible. These long wavelength VCSEL's now realize the low drivecurrent and ease of packaging benefits previously realized with shorterwavelength VCSEL structures and are a significant technical advance fromthe prior art shorter wavelength VCSELs. The new long wavelength VCSEL'shave been designed with efficient coupling into single mode fiber (SMF)in mind and have small, substantially circular emission spots in therange of 3 to 10 μm diameter and more preferably 6 to 7 μm diameter.These VCSEL devices also have a small numerical aperture (˜0.1) thatprovides the opportunity for precise alignment and direct butt couplingto single mode fibers in some circumstances. The near parallel opticalemission profile of these devices enables efficient optical couplinginto single mode fiber where the core diameter is typically 9 um.Furthermore, the long wavelength VCSELs have been specifically designedto operate in a single longitudinal mode at high power levels and arenow ready to replace the traditional Fabry Perot edge emitters commonlyused at long wavelengths such as 1300 nm.

The present invention brings together the new single mode longwavelength VCSEL devices with the previous work in offset launch torealize a novel and unique transmitter/transceiver device for multimodefiber links where offset launch with controlled mode conditioning isachieved without the use of a mode conditioning patchcord, and in someembodiments, without the use of any collecting or focusing elements.

Experimental evidence from testing of the present invention has beencollected and the results show that bandwidths considerably in excess ofthe overfill launch bandwidth of the multimode fiber has beendemonstrated, and in fact, GbE (1.25 Gb/s) data rates have beentransmitted over 2.8 km of multimode fiber, which is a factor of 5better than previously thought.

The ideal offset launch has been found experimentally to be 20 um on62.5 um MMF and this has been reliably achieved by translating the VCSELdevice laterally from the center axis of the multimode fiber by a fixedamount and then fixing the VCSEL in place. This is an easy process inmanufacture because of the VCSEL package configuration, and negates theneed for a mode conditioning patchcord. The emission size of the longwavelength VCSELs, which were developed for single mode fiberapplications, is fortuitously ideal for providing a restricted offsetlaunch on multimode fibers. This novel application of long wavelengthVCSEL technology to the problem of limited multimode fiber bandwidth nowenables the manufacture of very low cost transmitters capable ofdistances previously unattainable.

Accordingly, among the objects of the instant invention are:

-   -   the provision of a new way of achieving improved fiber bandwidth        on multimode fiber using long wavelength VCSEL devices;    -   the provision of a new launching technique which removes the        need for a mode conditioning patchcord specified in the        standards;    -   the provision of such a new configuration wherein the long        wavelength VCSEL could have a 1300 nm emission wavelength but        which could equally be any wavelength in the long wavelength        communications band (1100 nm to 1700 nm);    -   the provision of a low cost assembly technique for guaranteeing        the optimum offset launch condition into multimode fiber        systems; and    -   the provision of such techniques which enhance the bandwidth of        multimode fiber and therefore achieve longer transmission        distances than previously possible on multimode fiber, for        example, >2 km at GbE but whilst this is an example of what has        been achieved, other data rates and distances are possible (e.g.        10 GbE over 300 m).

Other objects, features and advantages of the invention shall becomeapparent as the description thereof proceeds when considered inconnection with the accompanying illustrative drawings.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

In the drawings which illustrate the best modes presently contemplatedfor carrying out the present invention:

FIG. 1 is an end view of a multimode fiber (MMF) showing the VCSELilluminating a spot offset from the optical axis of the MMF;

FIG. 2A is a schematic view of a first embodiment with a 1300 nm VCSELand lens coupled to the multimode fiber;

FIG. 2B is a schematic view illustrating exit of the transmission signalfrom the multimode fiber and collection of the signal by a wide anglephotodetector;

FIG. 3 is a cross-sectional view of a transmitter optical sub-assembly(TOSA) package constructed in accordance with the present invention;

FIG. 4 is a perspective view of a GigaBit Interface Converter (GBIC)Module including the TOSA package illustrated in FIG. 3;

FIGS. 5A through 5H are plan views showing assembly of the GBIC module;

FIG. 6 is a perspective view of multimode fiber pairs terminated with SCconnector plugs;

FIG. 7 is a perspective view of a communication system showingconnection of the GBIC module with a router device and connection of themultimode fiber pair with the GBIC;

FIG. 8 is a schematic view of a second embodiment with a 1300 nm VCSELbutt coupled to a multimode fiber;

FIG. 9 is a schematic view of a third embodiment with a 1300 nm VCSELand angular lens coupled to a multimode fiber; and

FIGS. 10A-10C are graphical illustrations of eye diagrams showing signalstrength at 2800 m for (A) Offset single mode VCSEL launch, (B) on axisFabry-Perot laser launch and, (C) 850 nm multimode VCSEL on axis launch.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, a multimode optical fiber communicationsystem including an offset launch long wavelength VCSEL transceiverconstructed in accordance with the present invention is illustrated andgenerally indicated at 10 in FIGS. 1 through 7. As will hereinafter bemore fully described, the instant multimode optical fiber communicationsystem is operative for communicating data between two remote nodes in acomputer network. Each node in the network includes a router devicegenerally indicated at 12 in FIG. 7. Each router 12 is typicallyconfigured to accept one or more transceiver devices 10 (See FIGS. 4-7)for transmitting data to and receiving data from another location(s).Each transceiver 10 includes a transmitter and a receiver operative witha pair of optical communication fibers 14 thus forming a bi-directionaldata link. The present invention is specifically directed to a noveloffset launch transmitter that permits two router devices 12 tocommunicate at higher speeds and over longer distances utilizing aninstalled base of multimode fibers 14.

Turning to FIGS. 4-7, there is shown a GBIC form factor communicationsystem. The GBIC form factor is standardized in the industry and is wellknown in the art. FIG. 4 illustrates a GBIC transceiver module 10comprising a housing generally indicated at 16 having a first end 18 anda second end 20 and a fiber connector structure 22 at the first end 18for directly receiving connectorized ends 24 of the multimode fibers 14.The GBIC form factor utilizes a standardized SC fiber connector systemas illustrated in FIG. 6. The individual fibers 14 are terminated with aferrule 26, an outer shroud 24 and a latching structure 30 forselectively engaging with the fiber connector structure 22 on thehousing 16 of the transceiver module 10.

Step-by-step assembly of the GBIC module 10 is illustrated in FIGS.5A-5H. The transceiver module 10 comprises upper and lower housing parts32, 24 respectively (FIGS. 4 and 5A), a latch structure 36 includinglatch tabs 38 at the first end 18 within the fiber connector structure22 (FIG. 5B), a circuit card 40 mounted within said housing 16 (FIG.5E), a transmitter optical subassembly 42 mounted to the circuit card 40adjacent the fiber connector structure 22 (FIGS. 5C-5E), a receiveroptical subassembly 44 mounted to the circuit board 40 adjacent thefiber connector structure 22 (FIGS. 5C-5E), and an electrical connector46 at the second end 20 of the housing 16 (FIG. 5E), the electricalconnector 46 being connected to the circuit card 40 and protruding fromthe second end 20 of the housing 16 for selectively connecting thecircuit card 40 with a mating receptacle (not shown) mounted inside therouter 12.

It is noted there are many form factors for optoelectronic transceiverconfigurations (GBIC, SFF, SFP, XFP etc), and those skilled in the artwill appreciate that the GBIC form factor is not critical to theinvention.

Referring back to FIG. 1, the multimode fibers 14, are part of aninstalled base of multimode fibers. Each of the multimode fibers 14comprises a core 48 and an external cladding layer 50. The multimodefiber 14 may typically have either a 50 μm core or a 62.5 μm core. InFIG. 1, the radius of the core 48 is indicated as R, the diameter of thebeam spot 52 is indicated as d, and the offset dimension of the beamspot 52 from the optical axis 54 of the multimode fiber 14 is indicatedas X.

The source of optical radiation for the offset launch transmittercomprises a single mode long wavelength VCSEL generally indicated at 56.For purposes of the present invention, long wavelength is generallydefined as operating in the 1300 nm optical communication window.However, it is to be understood that the present invention is applicableto all communication windows in the long wavelength band generallyranging from 1000 nm to 1700 nm.

Generally speaking, the VCSEL device 56 preferably has a beam spotdiameter d of approximately 6-7 μm. However, the beam spot 52 may rangein diameter from about 3 μm to about 9 μm. As discussed previously, thelong wavelength VCSEL's as utilized in the present invention wereoriginally designed for launching optical radiation into the smallercore (˜9 μm) of a single mode fiber. They are generally constructed toemit a small circular spot, as opposed to an elliptical spot for edgeemitters, and have a small (˜0.1) numerical aperture (NA), as opposed toa larger NA (˜0.4) for edge emitters. More specific details of the VCSELstructure and operation are disclosed in co-pending U.S. patentapplication Ser. No. 10/122,707 entitled “Long Wavelength VerticalCavity Surface Emitting Laser”, (U.S. Patent Publication No.2002/0150135) and Ser. No. 10/613,652 entitled “Method of Self Aligningan oxide aperture with an annular intra-cavity contact in a longwavelength VCSEL” (U.S. Patent Publication No. 2004/0058467), thecontents of which are both incorporated herein by reference.

Turning to FIGS. 2A and 2B, a schematic diagram of a first embodiment ofthe offset launch configuration is illustrated wherein the VCSELtransmission signal 58 is offset launched into the core 50 of themultimode fiber 14 with the help of a lens 60 to focus the laser output.The lens 60 operates to focus and size the launch beam 52 at the offsetlaunch point on the end face of the fiber 14. In this embodiment, boththe VCSEL 56 and the lens 60 are positioned offset from the fiber axis54.

The ideal offset launch has been found experimentally to be 20 um on62.5 um MMF and this has been reliably achieved by translating the VCSELdevice 56 laterally from the center axis 54 of the multimode fiber 14 bya fixed amount and then fixing the VCSEL 56 in place. This is an easyprocess in manufacture and negates the need for a mode conditioningpatchcord.

Turning to FIG. 3, the VCSEL 54 and lens 60 are assembled into a TO-38package generally indicated at 62. The TO-38 package 62 including aheader 64, and a can enclosure 66. The VCSEL 56 is mounted onto theheader 64 and provided with conventional electrical contacts 68 (Seealso FIGS. 5C-5G) that extend through the header 64 for connection withthe circuit card 40. The lens 60 is hermetically sealed at the top ofthe can enclosure 66. The VCSEL 56 is aligned with the optical axis ofthe lens 50 which defines the optical axis of the TO-38 package 62.

Still referring to FIG. 3, the transmitter optical subassembly 42comprises an annular base 70 having an upper surface 72 and a lowersurface 74, and a receptacle 76 having a first end 78 for receiving theconnectorized end 24 of a multimode fiber 14 and further having a secondend 80 which is received in assembled relation with the upper surface 72of the base 70. A weld sleeve 82 is affixed to the upper surface 72 ofthe base 70 for slidably receiving the second end 80 of the receptacle76. During assembly, the TO-38 package 62 is received within a recess 84in the bottom surface 74 of the base 70. The recess 84 is slightlylarger in diameter than the header 64 of the TO-38 package 62 so thatthe TO-38 package 62 can be laterally translated within the base 70 inthe x-y plane to provide the proper offset alignment with the core 48 ofthe multimode fiber 14 to be received in the first end 78 of thereceptacle 76. During this alignment, the TO-38 package 62 is alsoprovided with a slight angle (0-10°) along the z-axis to reduce backreflection of the optical radiation (transmission signal) off the endsurface of the multimode fiber 14. Once a rough alignment of the TO-38package 62 is completed, the TO-38 header 64 is secured to the base 70,and a fine alignment is completed by translation of the receptacleassembly 76 relative to the base 70.

Generally speaking, the VCSEL 56 and lens 60 are cooperativelypositioned within the receptacle 76 to direct the transmission signal 58onto the core 48 of the multimode fiber 14 offset from the optical axis54 of the core 48 when the connectorized transmitting end 24 of themultimode fiber 14 is received in the fiber connector structure 22.

The receiver optical subassembly 44 (FIG. 5C) is virtually identical tothe transmitter optical subassembly 42, with the exception of the VCSEL56 being replaced by a wide-angle photodetector 86 capable of collectingall of the transmission signal 58 exiting the terminal end of themultimode fiber 14 (See FIG. 2B). A separate illustration of thedetailed construction of the receiver optical subassembly 44 is notbelieved to be necessary for an understanding of the invention.

Experimental evidence has been collected which shows that bandwidthsconsiderably in excess of the OFL bandwidth of the MMF has beendemonstrated and in fact, GbE (1.25 Gb/s) data rate has been transmittedover 2.8 km of multimode fiber which is a factor of 5 better thanpreviously thought. In this regard, FIGS. 10A-10C are graphicalillustrations of eye diagrams showing experimental evidence of signalstrength at 2800 m for (A) offset single mode VCSEL launch (presentinvention), (B) on axis Fabry-Perot laser launch and, (C) 850 nmmultimode VCSEL on axis launch.

In a second embodiment as shown in FIG. 8, the VCSEL 56 is mountednormal to the end surface of the fiber 14, and the transmission beam 58is launched directly into the core 48 at an offset of approximately 20μm from the optical axis 54 of the core 48. In this particularembodiment, the transmission signal 58 is launched directly into thecore 48 of the multimode fiber 14 without an intermediate collectingelement, i.e. lens 60. More specifically, the TO-38 package 62 isprovided with a window (not shown) rather than the lens at the top ofthe can enclosure 66, and the TO-38 package 62 is mounted immediatelyadjacent to the end surface of the multimode fiber 14, i.e. eliminatingthe extra distance required for the focal length of the lens 60. Thisarrangement is advantageous for simplifying manufacture of the TO-38package 62 as there is no need to optically align a lens 60 with theVCSEL 56. Furthermore, the length of the entire transmitter opticalsubassembly 42 would be shortened considerably. A separate illustrationof the lenless TOSA 42 is not believed to be necessary for anunderstanding of the invention.

Turning to FIG. 9, a third embodiment is disclosed wherein the VCSEL 56and the lens 60 are positioned on axis and the launch beam is offsetlaunched at an angle into the core by directing the launch beam 58 at anangle from the lens 60. The transmitter optical assembly 42 in thisembodiment would be identical to that disclosed above in the firstembodiment. However, the optical axis of the TO-38 package 62 would befixed in alignment with the optical axis 54 of the core 48 of themultimode fiber 14, and then angled along the z-axis to provide both theoffset launch and the angled entry of the beam 58 into the core 48.

It is noted that the angular launch can also be achieved by buttcoupling the VCSEL in an orientation angled to the end surface of themultimode fiber. The angled butt couple launch thus provides a fourthalternative configuration.

It can therefore be seen that the present invention provides a uniqueand novel long wavelength transmission system that makes use of theexisting installed base of multimode fiber yet increases transmissiondistance and bandwidth. For these reasons, the instant invention isbelieved to represent a significant advancement in the art that hassubstantial commercial merit.

While there is shown and described herein certain specific structureembodying the invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and that the same is not limited to the particular forms hereinshown and described except insofar as indicated by the scope of theappended claims.

1. A transmitter for use in a multimode fiber communication system, saidtransmitter comprising: an optical source comprising a single mode, longwavelength vertical cavity surface emitting laser outputting opticalradiation in a substantially circular beam spot, said laser having asymmetric numerical aperture of about 0.1 and a beam spot diameter ofbetween about 3 μm to about 10 μm; and a collecting element forcollecting said optical radiation output from the laser and directingsaid optical radiation onto the end face of a multimode fiber, saidlaser and said collecting element being cooperatively positioned todirect said optical radiation onto the core of the multimode fiberoffset from the optical axis of said core.
 2. The transmitter of claim 1wherein said laser and said collecting element are cooperativelypositioned to direct said optical radiation at an angle to the end faceof said multimode fiber such that back reflection is reduced.
 3. Thetransmitter of claim 1 wherein said beam spot is about 6-7 μm indiameter.
 4. A transmitter optical subassembly for use in a multimodefiber communication system, said transmitter optical subassemblycomprising: a receptacle for receiving a multimode optical fiber; and atransmitter assembly including an optical source comprising a singlemode, long wavelength vertical cavity surface emitting laser outputtingoptical radiation in a substantially circular beam spot, said laserhaving a symmetric numerical aperture of about 0.1 and a beam spotdiameter of between about 3 μm to about 10 μm, and a collecting elementfor collecting said optical radiation output from the laser, saidreceptacle and said transmitter assembly being received in assembledrelation such that said laser and said collecting element arecooperatively positioned to direct said optical radiation onto the coreof a multimode fiber offset from the optical axis of said core of saidmultimode fiber when a transmitting end of said multimode fiber isreceived in said receptacle.
 5. The transmitter of claim 4 wherein saidlaser and said collecting element are cooperatively positioned to directsaid optical radiation at an angle to the end face of said multimodefiber such that back reflection is reduced.
 6. The transmitter of claim4 wherein said beam spot is about 6-71 μm in diameter.
 7. A transmitterfor use in a multimode fiber communication system, said transmittercomprising: an optical source comprising a single mode, long wavelengthvertical cavity surface emitting laser outputting optical radiation in asubstantially circular beam spot, said laser having a symmetricnumerical aperture of about 0.1 and a beam spot diameter of betweenabout 3 μm to about 10 μm said laser being positioned to launch saidoptical radiation directly onto the core of the multimode fiber offsetfrom the optical axis of said core.
 8. The transmitter of claim 7wherein said laser is positioned to launch said optical radiation at anangle to the end face of said multimode fiber such that back reflectionis reduced.
 9. The transmitter of claim 7 wherein said beam spot isabout 6-7 μm in diameter.
 10. A transmitter optical subassembly for usein a multimode fiber communication system, said transmitter opticalsubassembly comprising: a receptacle for receiving a multimode opticalfiber; and a transmitter assembly including an optical source comprisinga single mode, long wavelength vertical cavity surface emitting laseroutputting optical radiation in a substantially circular beam spot, saidlaser having a symmetric numerical aperture of about 0.1 and a beam spotdiameter of between about 3 μm to about 10 μm, said receptacle and saidtransmitter assembly being received in assembled relation such that saidlaser is positioned to direct said optical radiation onto the core ofthe multimode fiber offset from the optical axis of said core of saidmultimode fiber when a transmitting end of said multimode fiber isreceived in said receptacle.
 11. The transmitter optical subassembly ofclaim 10 wherein said laser is positioned to direct said opticalradiation at an angle to the end face of said multimode fiber such thatback reflection is reduced.
 12. The transmitter of claim 10 wherein saidbeam spot is about 6-7 μm in diameter.
 13. An optoelectronic transceivermodule for use in a multimode fiber communication system, saidtransceiver comprising: a housing having a first end and a second end; afiber connector structure at said first end of said housing for directlyreceiving connectorized ends of multimode fibers; a circuit card mountedwithin said housing; a transmitter optical subsassembly mounted to saidcircuit card adjacent said fiber connector structure, said transmitteroptical subassembly comprising a receptacle for receiving thetransmitting end of a first multimode fiber when a connectorizedtransmitting end of said first multimode fiber is received in said fiberconnector structure; an optical source comprising a single mode, longwavelength vertical cavity surface emitting laser outputting opticalradiation in a substantially circular beam spot, said laser having asymmetric numerical aperture of about 0.1 and a beam spot diameter ofbetween about 3 μm to about 10 μm; and a collecting element forcollecting said optical radiation output from the laser, said laser andsaid collecting element being cooperatively positioned within saidreceptacle to direct said optical radiation onto the core of said firstmultimode fiber offset from the optical axis of said core when saidconnectorized transmitting end of said first multimode fiber is receivedin said fiber connector structure; a receiver optical subassemblymounted to said circuit board adjacent said fiber connector structure,said receiver optical subassembly comprising a receptacle for receivingthe receiving end of a second multimode fiber when a connectorizedreceiving end of said second multimode fiber is received in said fiberconnector structure; and an optical receiver element said opticalreceivier being positioned within said receptacle to receive opticalradiation emitted from the core of said second multimode fiber when saidconnectorized receiving end of said second multimode fiber is receivedin said fiber connector structure; and an electrical connector at saidsecond end of said housing, said electrical connector connected to saidcircuit card and protruding from said housing for selectively connectingsaid circuit card with a circuit card assembly.
 14. The optoelectronictransceiver of claim 13 wherein said laser and said collecting elementare cooperatively positioned to direct said optical radiation at anangle to the end face of said multimode fiber such that back reflectionis reduced.
 15. The optoelectronic transceiver of claim 13 wherein saidbeam spot is about 6-7 μm in diameter.
 16. An optoelectronic transceivermodule for use in a multimode fiber communication system, saidtransceiver comprising: a housing having a first end and a second end; afiber connector structure at said first end of said housing for directlyreceiving connectorized ends of multimode fibers; a circuit card mountedwithin said housing; a transmitter optical subsassembly mounted to saidcircuit card adjacent said fiber connector structure, said transmitteroptical subassembly comprising a receptacle for receiving thetransmitting end of a first multimode fiber when a connectorizedtransmitting end of said first multimode fiber is received in said fiberconnector structure; an optical source comprising a single mode, longwavelength vertical cavity surface emitting laser outputting opticalradiation in a substantially circular beam spot, said laser having asymmetric numerical aperture of about 0.1 and a beam spot diameter ofbetween about 3 μm to about 10 μm, said laser being positioned withinsaid receptacle to direct said optical radiation onto the core of saidfirst multimode fiber offset from the optical axis of said core whensaid connectorized transmitting end of said first multimode fiber isreceived in said fiber connector structure; a receiver opticalsubassembly mounted to said circuit board adjacent said fiber connectorstructure, said receiver optical subassembly comprising a receptacle forreceiving the receiving end of a second multimode fiber when aconnectorized receiving end of said second multimode fiber is receivedin said fiber connector structure; and an optical receiver element saidoptical receivier being positioned within said receptacle to receiveoptical radiation emitted from the core of said second multimode fiberwhen said connectorized receiving end of said second multimode fiber isreceived in said fiber connector structure; and an electrical connectorat said second end of said housing, said electrical connector connectedto said circuit card and protruding from said housing for selectivelyconnecting said circuit card with a circuit card assembly.
 17. Theoptoelectronic transceiver of claim 16 wherein said laser and saidcollecting element are cooperatively positioned to direct said opticalradiation at an angle to the end face of said multimode fiber such thatback reflection is reduced.
 18. The optoelectronic transceiver of claim16 wherein said beam spot is about 6-7 μm in diameter.